How Many Monarchs in Mexico?
Estimating the Size of the Overwintering PopulationBack

Millions of Monarchs in Mexico
Monarchs migrate to Mexico from across eastern North America. They cluster by the millions in 12 mountain sanctuaries. Because the butterflies are so concentrated, scientists can measure the size of the entire migratory population. How count they count so many butterflies?

Measuring the AreaCovered With Monarchs
It's impossible to count individual butterflies, so scientists measure the area of forest covered with monarchs. This picture shows a monarch colony from the air. The trees look orange because they are covered with butterflies. How large an area does this colony cover? Count the trees and estimate the area.

Identifying Trees in the Colony
Scientists walk through the forest and decide which trees have enough butterflies to be considered part of the colony. They mark the edge of the colony with flagging tape.

Measuring the Border of the Colony
Next the scientists walk around the border of the colony and measure the distance between all of the marked trees. They must visit and measure all 12 of the monarch wintering sites. Their goal is to calculate the total area of forest covered with butterflies.

Measuring During Cold Temperatures
The scientists always measure the colonies in December or January. This is the coldest time of year, when the monarchs form their tightest clusters.

Historic Population Data
Population data have been collected consistently in Mexico since 1994. The graph shows the estimated area of forest the monarchs covered each winter. Look for trends. Why do you think it's valuable to collect population data every year?